Roots: Flip mentions Madonna's 'Frozen', Paloma Faith's 'Stone Cold Sober', and Vanilla Ice's 'Ice Ice Baby',; her monologue over who performed the song 'Cold As Ice' ("Whitesnake or Foreigner? No - M.O.P.!") references the latter's sampling of Foreigner's original version of the song. Goldilocks.

Goofs: Is there an entomologist in the house? The story gets some key terms wrong: the juvenile 'grub' stage of an insect like the Wirrn is called larva, not pupa (pupae tend to be immobile as they are undergoing metamorphosis); it's also likely that it is Wirrn antennae, not mandibles, which poke out of the frozen loch surface and are mistaken for branches.

How has word of 'forage porridge' as a viable food source succeeded in spreading as far as Nerva City without its disastrous repercussions also spreading? Did no-one think to run tests on it before eating or even touching it?

The Doctor's advice to Flip whilst attempting to fly a microlite make up a short list of how to send someone to their deaths, including staying in radio contact under adverse conditions and "aim for the gap in the clouds", - a phenomenon reputably known in aviation circles as a 'sucker's hole.'

Continuity: Forty years have passed since Nerva colonists returned to Earth. Pair bonding, the selective breeding control over the humans' gene pool, was abandoned due to 'low resources' and though they were not included Rohert voices the suspicion that gene stock from GalSec individuals has been introduced. There is mention of 'multitasks', suggesting a labour-oriented underclass possibly also created through the breeding programme. The Doctor notes that uniforms haven't changed much since his last encounter with Nerva's population. Lots of flora and fauna records have been lost, and weaponry includes fission guns. Toastie is nineteen years old and was five when her brother Iron died fifteen years ago (he sounds to Flip like a teenager.) As her voice and behaviour sounds closer to that of a ten year old, it could be assumed that Nerva colonists redefined the rate of one's aging based on rational universal physical constants, rather than orbital and rotational periods of Earth.

There are seven established colonies on Earth (these would have to be of a significant size to support the Olympics?), while Iron estimates a thousand transmit pads across the globe.. There are no artificial satellites, so radiophones must be used. Nerva Beacon, now only referred to as the "space station" is still orbiting Earth and at a far point of an ellipse after "thousands of years".

Earth's new capital is Nerva City, built on the former site of New York City (the name New York is no longer known.) It has a "Noah Centre" and stages the Olympics every quarter (either this is further evidence 'space timing' or they got their facts bungled from ancestral records. One year can't be every four Earth years, as Toastie would then be nearly eighty!) The Olympics doesn't appear to be simply a sporting event, as Dare plays a musical instrument touring the colonies for the Olympics. The temperature in Nerva City at the time of Flip and the Doctor's arrival on Loch Lomond is 20 degrees Celsius. The head of the population board is a man called Paul.

Inchfad is the northernmost outpost in Scotland. The first Nerva City colonists arrived at Loch Lomond fifteen years ago but were recalled shortly afterwards. A transmat substation on Ardlui serves Inchfad. GalSec colonists restocked the Loch with fish. Every colony was recalled fifteen years ago due to "climate change" (the Doctor suspects the radioactive winds were more likely the reason for the evacuation.) The temperature at Loch Lomond is minus fifteen degrees Celsius.

More than four hundred Wirrn sheltered in the Loch during the radiation and were trapped (but did not perish) when it froze over them. Veronica says the Wirrn haven't been seen for decades. The Doctor names Iron the new swarm leader. Wirrn land on planets for oxygen or to breed. Wirrn mucus, the slime trail left by Wirrn larvae (see: Goofs), is nicknamed "forage porridge" and used as a food source by the unwitting Nerva colonists.

Transmat technology is at its peak, and, according to the Doctor, the pinnacle of the most sophisticated and accurate technology Earth will ever develop. It works on a molecular level, locking onto "hadrons, quarks and the pinstripes of molecules." The Doctor credits the idea of using transmit to heat locations as his own.

The Doctor and Flip's arrival in Loch Lomond is an accident - Flip mistook the transmit pad for a phone booth. Travel by transmat makes Flip ill, but not the Doctor, who also doesn't appear to need a heated coat.

Flip has light brown hair.

Untelevised Adventures: The Doctor mentions that he has been to Loch Lomond before, but "a long time ago."

Links:The Ark in Space (Wirrn, pair bonding, The Doctor once again claims that humans are "indomitable."), Revenge of the Cybermen (Nerva beacon) and The Sontaran Experiment (GalSec colonists, mention of transmit globes, Sheer Jawn's South African accent is probably a reference) Flips refers to having "already died once" (The Fourth Wall) and mentions Napoleon (The Curse of Davros)

Welcome to Wirrn Isle, we hope you like technobabble! There are some glaring oversights (see: Goofs) in this tale, but the central idea, of an individual living on as a haunted consciousness inside a monster controlling it, is an appropriately chilling idea, and the Sci-Fi trappings this time are a gift. That said, episode three and four are a definite technobabble overload, and Flip's recklessness is laid on perhaps a little too thick. It's a good use of the Wirrn, though, and the South African accent was a nice nod to Nerva's TV origins.